Our current combination oak wood flooring-laid in 1970's addition-has served well for 20 years worth of necessary stackable washers. I'm told the sub-floor joists and support 2x6's running with them were placed close, not wide apart, but a crawl space prohibits me from directly seeing this. The stackable machines RPM's were max 800 however and our Neptune 2000 does walk of late as it's bearings are pretty gone and spin vibration has increased. New FL stackable washers are 1000 rpm for 3.5 cf drum or more. Weight of dryer (and in some way, weight of washer) may mitigate against movement. Direct drive FL motor spin technique apparently prevents knocking with spin start up I heard on a WP tech you tube.
I wish to ask if there is anyway to place on top of the current oak flooring an additional layer of wood or other to further firm up the floor and lesson vibration. The closet is 35" width x 32" depth x 84" height. The narrowest dimension for stackable placement is the door frame vertical lip at 29 & 1/4" which gives me pause on this idea in general as a weak link (can't floor into that area due to door).

I have spent hours researching the new machines, reading up on all the anti-vibration claims by W/D manufacturers. Users of these machines still report excessive vibration by some of these machines.

I found the forum commentary below on this topic very insightful. I'd appreciate your comments/advice if any additional anti-vibration wood or other surface could be added behind the front door and nailed down before purchase of a new higher 1000-1200 RPM stackable W/D. Thank you very much. I may cross post this on the laundry forum.

I realize I forgot the helpful discussion I mentioned so here it is.